1. Field of the Invention
The invention is a process for producing electronic devices in which ultraviolet curable substances are used to produce protective coatings on the devices.
2. Description of the Prior Art
Resins cured by ultraviolet radiation are extensively used in electronics applications. Typical uses are etch resist, plating resist, solder masks and protective coatings. These resists may be used in various stages in the fabrication procedure including at the conclusion of the fabrication procedure to protect the device in use. One particular application of interest is that of a protective coating for flexible printed circuitry. When used in this way as well as in other ways it minimizes the effects of heat, moisture, oxygen, miscellaneous particulate contaminants and mechanical damage to the electronic device. Curable resins are also used in certain soldering applications such as wavesoldering to protect conductors adjacent to that being soldered from the molten solder. This is particularly important in circuits having narrowly spaced, densely packed, conductor geometries which is the current design trend.
Although many polymers are curable in the pure form, it is often desirable to add sensitizers which promote rapid curing in a convenient part of the electromagnetic spectrum. Particularly convenient, especially for some of the newer classes of compounds used as photosensitizers, is the spectral region around 3,000 Angstroms since such sensitized polymer mixtures provide good stability to ambient conditions but provide rapid curing on exposure to curing radiation (see for example, "New Photosensitive Systems for Presensitized Printing Plates" by J. Poot et al., in J. Photographic Science, Vol. 19, p. 88 (1971)). Similarly, certain benzoin compounds have greater quantum yields at 313 nm than 366 nm (see "The Photochemistry of Benzoin Ethers" by Ashok Chattopadhyay, University Microfilms, 1974).
From a processing point of view, a rapid, low temperature coating procedure is highly desirable. It prevents damage to heat sensitive parts of electronic devices and is economically compatible with other processing steps in the fabrication of these devices especially where roll-to-roll continuous processing of flexible circuitry is being used. For many applications, patterns of cured resins must be laid down on the electronic circuit. Tolerances in the pattern shape are often quite high, especially in circuits having narrowly spaced, densely packed conductor geometries. It is this combination of rapid and complete curing at room temperature together with the high pattern definition which proves difficult to accomplish under practical manufacturing conditions. Also, the resin should be curable with ultraviolet radiation but relatively stable to visible radiation.
An appreciation of the difficulty in producing satisfactory cover coatings under manufacturing conditions may be obtained by summarizing some obvious requirements of the coatings. The coating should be transparent so as to allow inspection of the covered conductors and other circuit elements. It should be flexible to permit use on flexible substrates. The coating should withstand immersion in molten solder to permit wavesoldering and withstand organic solvents used to remove flux from circuits following the solder operation. It should be flame retardant, have no adverse (e.g., corrosive) effects on circuit elements such as conductors and no adverse effects on the circuit's electrical properties such as resistance, breakdown voltage, losses, semiconductor properties, etc.
Typical procedures presently used involve the use of a mask mounted on pyrex or glass to produce a pattern and the use of pyrex or glass lenses to concentrate the ultraviolet radiation sufficiently to produce curing. Although this procedure is often satisfactory, increasing processing speeds and more rapid curing is often highly desirable. The use of glass or pyrex attenuates the intensity of the ultraviolet radiation especially at the wavelengths most effective in curing many cover coats. This often limits the rate at which coating can be cured and poses problems concerning the dissipation of the heat generated from the absorbed radiation. Although this rate limitation is not disadvantageous under many conditions, where very high speed processing is desired, it may limit processing speed. The use of quartz lenses and mask substrates might permit higher processing speeds but leads to much greater expense and lenses of lower optical quality.